Book Lists

Database of Award-winning (children's) literature
GoodReads
Read-4-teens
Winners of the Nobel Prize for Literature
Winners of the Pulitzer Prize for Fiction
Winners of the Booker Prize
100 Great works of Science Fiction
List to book lists (award winners and others) from Minuteman Libraries (Brookline and others)

Lots of lists--
ATN Lists of lists--includes by genre, characters, issues, etc. Huge list of lists
ATN Wiki--an even more up-to-date set of lists, with an "if you liked , you might like section; also lists by genres

If you liked Percy Jackson, you might like...

Detective/Crime Fiction

Classic Crime fiction (and lots of other useful suggestions as well)

Amazon.com Lists:

Great Reads for Teens
Another list: Superb Reads for Teens
NY Public Library Sci. Fi. list for Teens
Great Books for Teen Age Readers
Fantastic Teen Books

Interactive book suggestion sites:

WhichBook--
What Should I Read Next?--This site makes suggestions based on a book you enjoyed reading
Brookline Public Library readers' advisory service and resources

BookFlavor--a site that recommends lots of books based on the interests (topic, genre, etc.) that you input.